The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for preventing a belt from slipping on pulleys of the belt drive transmission.
A known control system for a continuously variable belt-drive transmission disclosed in Japanese Patent Application Laid-Open No. 54-157930 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
A rotation speed sensor in the form of a pitot tube is provided adjacent the drive pulley to measure the speed of the oil in an annular groove formed on the periphery of the drive pulley so that a pitot pressure which is proportional to the engine speed is detected. The transmission ratio control valve is applied with the pitot pressure at one end and with a spring load dependent on accelerator pedal depressing degree at the other end.
Thus, the transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed, that is, pitot pressure. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the engine speed exceeds a predetermined value, an electromagnetic clutch engages, so that the vehicle is started. When the vehicle speed and engine speed exceed set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced, even if the engine speed is kept constant. Thus, the vehicle speed increases with the reduction of the transmission ratio.
The electromagnetic clutch is automatically disengaged in dependency on engine speed or vehicle speed to prevent the stall of the engine. In a steady state, the clutch is engaged so that the drive pulley is directly connected to the engine. Accordingly, sufficient pitot pressure is generated to control the transmission ratio.
To the contrary, the pitot pressure is reduced when the wheels are locked as a result of a sudden braking on a road having a low friction coefficient such as a snowy road. Such a phenomenon is explained with reference to FIGS. 9a to 9e.
When the wheels are locked, the wheel speed Vw rapidly decreases to zero, while the vehicle speed Vm does not quickly decrease because of the slipping of wheels on the road. Since a vehicle speed sensor is provided to detect the speed of an output shaft of the transmission, the vehicle speed sensor produces a signal which means the vehicle speed largely decreases. As a result, the electro magnetic clutch is disengaged, and the speed of the drive pulley largely reduces. Accordingly, the pitot pressure Pt and hence primary pressure Pp in an oil chamber of the servo device of the drive pulley rapidly decrease. Since the drive and driven pulleys do not rotate, the belt is held by the gripping operation of the pulleys at a position where the pulleys stop, providing a small transmission ratio i. Since the pitot pressure Pt is zero, the transmission ratio control valve is in a large transmission ratio providing state. Accordingly, the hydraulic system intends to shift the belt and pulley device to the large transmission ratio position. However, since the belt is gripped by the pulleys, positions of the pulleys and belt can not be shifted. Thus, the transmission is held at the large transmission ratio position.
When the brake pedal is released, the wheels are unlocked so that the wheel speed is quickly increased to coincide with the vehicle speed Vm. Therefore, the driven pulley is driven by the wheels to rotate the drive pulley through the belt. When the pulleys and belt start to rotate, the gripping force reduces, so that the belt and pulley device is quickly shifted to the large transmission ratio position. Since the inertial mass of a driven member of the electromagnetic clutch, the drive pulley and other members provided between the clutch and the drive pulley is large, a large primary pressure is necessary to grip the belt so as to rotate the drive pulley, the driven member and others. However, since the pitot pressure is still low, the primary pressure Pp is low. As a result, the belt slips on the drive pulley, causing damaging of the belt. In addition, the drive pulley is gradually rotated, so that the increase of the pitot pressure delays. Consequently, the transmission ratio is not accurately controlled.